Adsorption and photo-oxidation of 3,4-dihydroxy-cinnamic acid on TiO2 films

The photo-oxidation of 3,4-dihydroxy-cinnamic acid (DHCA) is investigated on TiO2 films prepared from commercial Degussa P-25 (FP25 films) and from colloidal TiO2 prepared by the sol–gel method (FSG films). Photo-oxidation rates have been found to be inversely correlated to the amount of adsorbed species. The extent of adsorption increases as the solution pH increases from 1 to 4 and is more pronounced in the case of FP25 samples for which the amount of adsorbed species is 2.7 × 10−10 mol cm−2, i.e. about threefold higher than for the FSG case. Analysis of DHCA adsorption on TiO2 suspensions provided information complementary to that collected on films. Diffuse reflectance UV–visible and FTIR spectra of adsorbed DHCA are conspicuously different for the two types of films.